The host range of and host-cell susceptibility to the avian RNA tumor viruses is dependent on specific interactions between genetically controlled properties of the viral envelope and the host-cell plasma membrane. The cell membrane "receptor" may act as an inherited isoantigen and be detectable by existing serological methods. This proposal is aimed at the serologic identification of erythrocyte isoantigens which may be associated with three different autosomal loci, each controlling susceptibility to a different viral subgroup. Lines of chickens which have been extensively blood typed for eleven known blood group systems will be crossed with an inbred line fully characterized for susceptibility or resistance to the three viral subgroups. Chickens matched for known blood type but differing in susceptibility will be used as donors and recipients for immunizations with erythrocytes. The resulting antisera will then be tested to determine if the antigen they detect is genetically, immunologically or functionally associated with the plasma membrane site for viral penetration.